1. Field of the Invention
The present invention relates to liquid crystal display devices, portable terminal devices and projector devices using the same, and methods and programs for driving the liquid crystal display device, in particular, to a field sequential type active matrix liquid crystal display device, a portable terminal device and a projector device using the same, and a method and a program for driving the liquid crystal display device.
2. Description of the Related Art
Active matrix type liquid crystal display devices including thin film transistors (TFT) at each of the pixels are capable of displaying videos with high picture qualities, so that many of such type of display devices are used for thin-type television sets, displays for portable terminal devices, projector light valves, etc. Such liquid crystal display device used for thin type television sets and portable terminal devices normally has such a structure that is shown in FIG. 31. In this liquid crystal display device, for displaying a color image, one pixel is divided into three sub-pixels, and color filters of red (R) 91, green (G) 92, and blue (B) 93 are provided to the sub-pixels, respectively. Gate line 97 are scanned by a gate driver 94, and video signals are supplied to data lines 98 by a data driver 95 to select pixels so as to drive corresponding liquid crystals to achieve a color display.
In the meantime, a light valve used for a liquid crystal projector has a structure where each pixel is formed as a single piece as shown in FIG. 30. There is no color filter provided in the liquid crystal display device that configures this light valve, and a single pixel is not divided into a plurality of sub-pixels, either. This is because, in a typical projector, three light valves of red (R), green (G), and blue (B) are used for corresponding to light of three primary colors, gate lines 87 are scanned by a gate driver 84, and video signals are supplied to a data line 88 by a data driver 85 to drive the liquid crystals of each pixel.
It is necessary to divide a single pixel into three sub-pixels in the liquid crystal display device which performs color displays by using the color filters, as it is described earlier by referring to FIG. 31. Thus, when the resolution of the liquid crystal display device is increased, the areas of each of the sub-pixels become reduced. This leads to reduction of the numerical aperture, which results in causing light loss. Further, the three-plate type liquid crystal projector shown in FIG. 30 requires three light valves, so that the cost thereof becomes high and the device cannot be formed small in size.
As a measure for overcoming such issues, there is a field sequential type liquid crystal display device as described in U.S. Pat. No. 5,920,298 (FIG. 8) (patent document 1). The field sequential type is a system which divides the time for a liquid crystal display device to display a video of one screen into three periods, displays videos corresponding to colors of red (R), green (G), and blue (B) in each period, and switches the colors of the light irradiated to the liquid crystal display device by synchronizing with the videos to achieve a color display. As described above, the field sequential type which performs color display by switching the color of the light source, without dividing each pixel of the liquid crystal display device into sub-pixels of red (R), green (G), and blue (B), can realize a liquid crystal display device having improved light utilizing efficiency, since color filters are not used and the numerical aperture of the pixel can be designed large.
FIG. 29 shows one example of a configuration of the field sequential type liquid crystal display device. This liquid crystal display device is configured with a back light (BL) 104 that can switch the color of light to be irradiated to a display device body 103 to red (R), green (G), and blue (B); a display device body 103; a control circuit 102 for controlling the display device body 103 and the back light 104; and a signal source 101. FIG. 32 shows one example of a configuration of the display device body used in such system. The display device body includes: a pixel matrix in which pixels each including a pixel TFT, a pixel capacitance and an accumulative capacitance are disposed in matrix at each intersection point of the data lines 118 and the gate lines 117 arranged vertically and laterally; and a data driver circuit 115 disposed in the periphery of the pixel matrix for driving the data lines 118 as well as a gate driver circuit 114 disposed in the periphery for driving the gate lines 117.
Actions of the field sequential type liquid crystal display device will be described by referring to a timing chart of FIG. 33. A frame period Tf during which a video for one screen is displayed in the liquid crystal display device is divided into three sub-frame periods Tsf_r, Tsf_g, and Tsf_b. In the sub-frame period Tsf_r, executed is an action for displaying a video of red (R) at each pixel of the liquid crystal display. First, a gate line G1 is set to high level. Synchronously with this, a video signal is written to the data lines D1-D10. Thereby, the video signal is written to each pixel on a pixel row that is connected to the gate line G1. By performing this action for all the gate lines G1-G8, the video signal of red (R) is written to all the pixels.
After writing the video signal of red (R) to all the pixels, the light source of red (R) is lighted up after a passage of a certain waiting period. With this, the liquid crystal display device displays a video of red (R) among a color video. LED R herein indicates a control signal for lighting up the light source of red (R). In the same manner, a video of green (G) is displayed in Tsf_g, and a video of blue (B) is displayed in Tsf_b. Thereby, observers mix the colors of those videos in terms of time to recognize it as a color video.
However, unless the response speed of the liquid crystal is extremely high in the field sequential type, luminance unevenness and lowering in luminance occurs in the screen. The reasons for the cause of such problem will be described below.
Regarding T1 and T8 in FIG. 33, T1 shows changes in the transmittance of the pixels that are connected to the gate line G1, and T8 shows changes in the transmittance of the pixels that are connected to the gate line G8. If the waiting period is too short and the light source is lighted up while the transmittance change of T8 is still continuing, there is a difference generated between the luminance within a screen even though the same luminance is to be displayed on the entire screen. Meanwhile, if the waiting period is set too long, the light-up time of the light source becomes too short, thereby resulting in providing a dark display. Therefore, it is necessary for the field sequential type liquid crystal display device to use a liquid crystal material that is capable of enabling sufficient response within a sub-frame period and capable of operating at a very high speed, that is, at a response speed of the liquid crystal of lower than or equal to a few ms.
As a measure for solving such problem, a method of applying a voltage, which applies a large electric field to the liquid crystal, to counter electrodes during the period of writing the video signal for one screen to the liquid crystal display device, and changing the voltage of the counter electrodes after completing the writing to simultaneously change the liquid crystals of all the pixels of the liquid crystal display device to the state corresponding to the video signal thereby eliminating the luminance difference in the screen is proposed from Macknight (patent document 1).